THE PHENOMENON OF CONTRACTION. 57 



onds to cause tetanus.* A distinguishing and important charac- 

 teristic of the plain muscle is its power to remain in tone, that 

 is, to remain for long periods in a condition of greater or less con- 

 traction. Doubtless this tonic contraction under normal relations 

 is usually dependent upon stimulation received through the ner- 

 vous system (neurogenic tonus), but the muscle, when completely 

 isolated from the central nervous system, whether in or out of 

 the body, continues to exhibit the phenomenon of tone to a 

 remarkable degree. In most of the organs in which plain muscle 

 occurs there are present also numerous nerve cells, and it is 

 therefore still a question as to whether the tonic changes shown 

 by this tissue, after separation of its extrinsic nerves, depend 

 upon a property of the muscle itself (myogenic tonus) or upon 

 their intrinsic nerve cells. Most observers adopt the former 

 view. The importance of this property of tone in the plain 

 muscle tissues will be made fully apparent in the description 

 of the physiology of the organs of circulation and digestion. 

 We shall find that constantly throughout life the walls of the small 

 arteries resist a high internal pressure, and that in other visceral 

 organs pressures of varying amounts are supported by the tonicity 

 of the plain muscle in their walls. Many interesting observations 

 in late years tend to show that the tension developed in plain muscle 

 in a state of tone is accompanied by little or no production of heat 

 and by little evidence of chemical changes of an oxidative nature. t 

 In some very economical way, so far as the consumption of mate- 

 rial and energy is concerned, the condition of the muscle may be 

 changed from a state of little tone to one of greater tone. Plain 

 muscle may exhibit also the phenomenon of rhythmical activ- 

 ity that is, under proper conditions it may contract and re- 

 lax rhythmically like heart tissue. { Such movements have 

 been observed and studied upon the plain muscle of the ureter, 

 the bladder, the esophagus, stomach, and other portions of the 

 alimentary canal, the spleen, the blood-vessels, etc. This property 

 seems to be very unequally distributed among the different kinds 

 of plain muscle found in the same or different animals, but this 

 fact serves only to illustrate the point already sufficiently empha- 

 sized, that grouping one kind of tissue e. g., plain muscle into 

 a common class does not signify that the properties of all the mem- 

 bers of the group are identical. The question as to how far the phe- 



* Schultz, "Zur Physiologie der langsgestreiften (glatten) Muskeln," 

 "Archiv f. Physiologie," suppl. volume, 1903, p. 1. See also Stewart, "Amer- 

 ican Journal of Physiology," 4, 185, 1900. For finer histology see M'Gill, 

 " American Journal of Anatomy," ix, 1909. 



t Snyder, "American Journal of Physiology," 35, 340, 1914. 



t Engelmann, "Archiv f. d. ges. Physiologie," 2, 243, 1869. Stiles, "Amer. 

 Jour, of Physiology," 5, 338, 1901. 



